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jdh321 11-12-2007 10:01 PM

Questions about major voltage drop.
Here is my situation. I am in the process of wiring an outbuilding. Its about 100 feet from my house. A few mnths ago, I wired a new circuit for two outdoor outlets. I ran the lead for the out building off of the GFCI outlet. Anyway, there is about 100 feet of outdoor (dont remeber the actual name of it) 12/2 wire. Unfortunately it is already covered. Anyway, the voltage after the 100 ft run fluctuates from 70 to 100 volts. This is far more voltage drop than any calculator gave me. Any thoughts on what is causing this. I really dont want to have to dig up the #12 wire and replace it with #10. If I out any load on the wire it trips the GFCI. Thanks in advance.

goose134 11-12-2007 10:50 PM

I am going to assume that when you say you ran 12/2 that it had a ground. That aside, it sounds like you nicked a conductor in there somewhere along the line. Probably the neutral. When you dig it up run #10. 100' would justify it. Also, you could try tying into the line side of the GFI and putting another GFI rec. in the outbuilding.

Stubbie 11-12-2007 10:57 PM

For what its worth a 20 amp 120 volt circuit should have had at least 10 awg and calls for #8 awg copper.

Anyway your voltage drop is excessive for no load and if your gfci is tripping on load you have a ground fault in a wire. It isn't a hot to equipment ground or hot to neutral or the gfci would trip and stay tripped or the circuit breaker would trip. So most likely a neutral to ground fault. Soon as you apply a cord connected load to the circuit the resulting neutral current is leaking at the fault point and the gfci sees the imbalance between the hot and the neutral and trips. You might check your receptacles to see if anything is touching the bare of the white wire other than that go get your shovel......:)

Remember to that the voltage drop is going to be measured from your panel not the the gfci you took power from. So if the gfci is 50 feet from the panel your distance is really 150 feet one way....300 feet round trip.

Andy in ATL 11-13-2007 03:30 PM

What do you mean in your first sentence, Stub??? Do you mean for his distance he needs #10 or #8???


Stubbie 11-13-2007 03:59 PM

# 10 is acceptable voltage drop for 20 amps # 8 is slightly better than recommended voltage drop for 20 amps. #12 will only be acceptable if the load is 15 amps or less. #10 will put 115 volts at 100 feet, This is the minimum listed voltage rating for most 120 volt power tools.

I would go with #10 for ease of connection to receptacles. I never worry about that 5 and 3 percent stuff on feeders and branch circuits. I size wire to get 115 volts minimum to the outlets. (theres that word again....:))


jdh321 11-13-2007 05:11 PM

I did the dig
Okay guys, thanks for the info, even if it wasnt what I wanted to hear. I got some #10 wire and some 1/2" conduit. Do I need to have a grounding stake at the outbuilding or can I just contiue the ground from the house.

Stubbie 11-13-2007 07:50 PM

Are you still just planning on having a switch for a light and a couple receptacles? You are not installing a sub-panel as I understand you.

Still planning on using the load side of a gfci to get the power for the wiring run to the outbuilding?

Your wire for the conduit needs to be rated for wet location like thwn. To be code compliant it needs to be black for hot, white for neutral or green or bare for ground.

If it is a cable like 10/2 G it needs to be uf 10/2 G not nm-b. i am assuming you have individual wire for conduit like thwn. Otherwise direct burial cable should just be exactly that... direct buried... and not in conduit, though there is nothing that says you can't. It would be too large for 1/2" conduit anyway as it would exceed the 53% fill rating.

If you use the gfci you can bury the conduit at 12" otherwise 18 inches. No ground rod is required at the building if only one 20 amp circuit at 120 volts.

You are confusing the grounding rod with the equipment ground wire in the conduit. These serve different purposes.

You are required to have the branch circuit hit a switch to operate a light and have at least one gfci protected receptacle. The gfci at the house would satisfy this as it will protect all downstream devices.

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